bsd-user/signal.c: Implement signal_init()

bsd-user/signal.c: Implement signal_init()

Initialize the signal state for the emulator. Setup a set of sane
default signal handlers, mirroring the host's signals. For fatal signals
(those that exit by default), establish our own set of signal
handlers. Stub out the actual signal handler we use for the moment.

Signed-off-by: Stacey Son <sson@FreeBSD.org>
Signed-off-by: Kyle Evans <kevans@freebsd.org>
Signed-off-by: Warner Losh <imp@bsdimp.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org> XXX SIGPROF PENDING